I make my screened bottom boards with butt joints using cheap firring strips, 1x2 for the sides, back and back bottom and 1x4 for the landing board. By having the sides and back standing up, and the back bottom and landing board laying down I get a 3/4" height of the hive bodies above the SBB floor. I use gorilla glue and screws to hold everything together. Costs me just a few bucks and maybe 30 minutes to make.
Quick ASCII drawing top view:
|---------| Sides and back are 1x2 standing up
|=====| Back bottom is 1x2 laying down
|+++++|Staple screen to back bottom, landing and side rails
|=====|Landing board 1 1x4 laying down
The bottom of all boards are flush to each other.
Note, by having the back the full width, 16.25, and the sides shorter (21.25") butt joint to the back, and the back bottom board, 15.5", butts to the sides making for a very strong interlocking joint overall. You could just make the sides 22" and have all of the cross boards at 15.5" and be butted to the sides. It probably does not make much difference, it's not like the bottom board takes a lot of stress.
Third year underway, been a fascinating and fun journey so far. Have learned enough to know I have much more to learn.